I am astonished that nobody has provided this answer yet. I've been into online gaming for the better half of my life and can tell you with fair certainty that most German online gamers will understand the term
Nap
(say "nup") or in spoken context
Nappl
(say "nupil")
Example of use:
Du bist so ein Nap! (= "You are such a noob")
(= "You are such a noob")
I can even provide a source for my claim: http://www.urbandictionary.com/define.php?term=nap (way down)
"Etymologically" it seems to come from the way the words are spoken: "noob" would be written as "nub" in German and "nub" would be pronounced like the German written word "nab", eventually arriving at the word "Nap". Seems like a long shot, but that's internet creativity in action ;)
Already mentioned synonyms that fit your question are "Boon" or the more derogatory "Kackboon" (say "cuck-boon").